The present invention relates generally to floating platform systems for testing and producing hydrocarbon formations found in deep (600-10,000 feet) offshore waters, and in deeper or shallower water depths where appropriate, particularly to a method and system for economically producing relatively small hydrocarbon reserves in mid-range to deep water depths which currently are not economical to produce utilizing conventional technology.
Commercial exploration for oil and gas deposits in U.S. domestic waters, principally the Gulf of Mexico, is moving to deeper waters (over 600 feet) as shallow water reserves are being depleted. Companies must discover large oil and gas fields to justify the large capital expenditure needed to establish commercial production in these water depths. The value of these reserves is further discounted by the long time required to begin production using current high cost and long lead-time designs. As a result, many smaller or xe2x80x9clower tierxe2x80x9d offshore fields are deemed to be uneconomical to produce. The economics of these small fields in the mid-range and deep water depths can be significantly enhanced by improving and lowering the capital expenditure of methods and apparatus to produce hydrocarbons from them. It will also have the additional benefit of adding proven reserves to the nation""s shrinking oil and gas reserves asset base.
In shallow water depths (up to about 300 feet), in regions where other oil and gas production operations have been established, successful exploration wells drilled by jack-up drilling units are routinely completed and produced. Such completion is often economically attractive because light weight bottom founded structures can be installed to support the surface-piercing conductor pipe left by the jack-up drilling unit and the production equipment and decks installed above the water line, which are used to process the oil and gas produced from the wells. Moreover, in a region where production operations have already been established, available pipeline capacities are relatively close, making pipeline hook-ups economically viable. Furthermore, since platform supported wells in shallow water can be drilled or worked over (maintained) by jack-up rigs, shallow water platforms are not usually designed to support heavy drilling equipment on their decks. This enables the platform designer to make the shallow water platform light weight and low cost, so that smaller reservoirs may be made commercially feasible to produce.
Significant hydrocarbon discoveries in water depths over about 300 feet are typically exploited by means of centralized drilling and production operations that achieve economies of scale. For example, production and testing systems in deep waters in the past have included converting Mobile Offshore Drilling Units (xe2x80x9cMODU""sxe2x80x9d). into production or testing platforms by installing oil and gas processing equipment on their decks. A MODU is not economically possible for early production of less prolific wells due to its high daily cost. Similarly, early converted tanker production systems, heretofore used because they were plentiful and cheap, are also not economical for less prolific wells. In addition, environmental concerns (particularly in the U.S. Gulf of Mexico) have reduced the desirability of using tankers for production facilities instead of platforms. Tankers are difficult to keep on station during a storm, and there is always a pollution risk, in addition to the danger of having fired equipment on the deck of a ship that is full of oil or gas liquids.
TLP""s have attracted considerable attention in recent years. A conventional TLP consists of a four column semi-submersible floating substructure, multiple vertical tendons attached at each corner, tendon anchors to the seabed, and well risers. A variation of the conventional TLP, a single leg TLP, has four columns and a single tendon/well riser assembly. The conventional TLP deck is supported by four columns that pierce the water plane. These types of TLP""s typically bring well(s) to the surface for completion and are meant to support from 20 to 60 wells at a single surface location. In a mono-column TLP, risers for subsea wells can be hung on the outer surface of the column. In some designs where the TLP column is provided with a moonpool, the well risers are hung about the periphery of the moonpool. In U.S. Pat. No. 5,330,293, a platform is disclosed having a large moonpool. The well risers are horizontally secured in stanchions located about the periphery of the moonpool. The well risers are permitted to move vertically but not horizontally because of the restraint of the stanchions.
There continues to be a need however for improved platform and drilling systems, particularly for use in deep waters. As the water depth increases, the greater the load the platform must support. Thus, larger platform hulls are required to support the increased load and thereby increasing the cost of the platform. Another factor adding to the cost of a platform is riser spacing. If greater riser spacing is required, as for example to compensate for riser deflection in high current environments, platform size and cost may be driven by riser spacing rather than payload. Thus, minimizing riser spacing requirements would be highly desirable for reducing the size of the platform and reducing the platform cost.
It is therefore an object of the present invention to provide a floating platform system which suppresses substantially all vertical motions. A single large column provides buoyancy more efficiently than multiple columns with a small water plane area.
It is another object of the invention to provide a floating platform system having a central column wherein top-tensioned vertical production and drilling risers traverse the platform hull in a central moonpool.
It is yet another object of the invention to provide a floating platform system wherein minimum the well riser spacing requirements by providing lateral riser restraint and a lowering or pull-down system for running risers.
The present invention provides a floating platform for producing and processing well fluids produced from subsea hydrocarbon formations. The platform supports one or more decks above the water surface for accommodating equipment to process oil, gas, and water recovered from the subsea hydrocarbon formation. In a preferred embodiment, the platform includes a central column substantially located below the water surface and in the wave zone. The upper portion of the central column extends above the water surface. The central column includes a base structure comprising three or more pontoons extending radially outwardly from the bottom of the central column. The platform is anchored to the seabed by one or more tendons secured to the base of the central column. A moonpool open at the upper and lower ends of the central column extends axially through the central column. A riser lateral restraint system is supported within the moonpool.